Well, after two years it was about time to answer the question.
I had the same issue.
File is located in /components/com_virtuemart/sublayouts/customfield.php
In case of overrides it can also be in /templates/your_template/html/com_virtuemart/sublayouts/customfield.php
Line 634 of the file: $customfield->display = 'customfield related product has no value';
Just set to '' or anything you like.